This invention relates to a process for the manufacture of xcex1, xcfx89-diiodoperfluoroalkanes from the reaction of tetrafluoroethylene and 1,2-diiodoperfluoroethane with periodic removal of the byproduct perfluorocyclobutane.
It is known that xcex1, xcfx89-diiodoperfluoroalkanes, represented by the formula Ixe2x80x94(CF2CF2)nxe2x80x94I, where n is an integer between 2 and 6, are useful as reagents in the synthesis of various fluorochemicals and fluoropolymers. For example, Carlson et al. (U.S. Pat. No. 5,214,106) disclose fluoroelastomers having terminal iodo groups formed by polymerization in the presence of one or more of these xcex1, xcfx89-diiodoperfluoroalkanes.
In general, the manufacture of xcex1, xcfx89-diiodoperfluoroalkanes from iodine and tetrafluoroethylene (TFE), or from 1,2-diiodoperfluoroethane and TFE requires a high reaction temperature ( greater than 240xc2x0 C.) and a high TFE pressure ( greater than 3.1 MPa) for reasonable conversion and yield (i.e. for less than 40 mole % of unreacted 1,2-diiodoperfluoroethane remaining in the product mixture).
Bedford and Baum (J. Org. Chem. 1980, 45, 347-348) disclose a laboratory-scale method for preparing xcex1, xcfx89-diiodoperfluoroalkanes by the telomerization of iodine and tetrafluoroethylene. However, the yield and conversion were quite poor.
Suzuki et al. (JP 61-31084) disclose the manufacture of 1,4-diiodoperfluorobutane (and higher order diiodoperfluoroalkanes) by the thermal decomposition of 1,2-diiodoperfluoroethane with the pre-addition of iodine and an inert gas to the reactor. However, conversion is still rather low and the resulting product must be separated from a large amount of iodine.
It is an object of the present invention to manufacture xcex1, xcfx89-diiodoperfluoroalkanes under relatively mild conditions in a process having relatively high conversions and wherein minimal amounts of iodine must be separated from the diiodoperfluoroalkanes produced.
Applicants have developed a relatively low temperature and low pressure process, having acceptable conversions, for the manufacture of diiodoperfluoroalkanes. Accordingly, an aspect of the instant invention is a process for preparing xcex1, xcfx89-diiodoperfluoroalkanes comprising the steps of:
A. heating 1,2-diiodoperfluoroethane in a reactor to a temperature between 200xc2x0 and 240xc2x0 C.;
B. adding to said reactor a quantity of tetrafluoroethylene so as to result in a total pressure in said reactor of between 1.7 and 3.4 MPa;
C. maintaining said temperature and pressure for a period of time to form a mixture of xcex1, xcfx89-diiodoperfluoroalkanes and perfluorocyclobutane;
D. cooling said mixture to a temperature below 75xc2x0 C. and then discharging gaseous perfluorocyclobutane and unreacted tetrafluoroethylene from said reactor, leaving a liquid mixture in said reactor;
E. heating said liquid mixture to a temperature between 160xc2x0 and 235xc2x0 C.; and
F. repeating steps B)-E) until a desired mixture has been prepared of xcex1, xcfx89-diiodoperfluoroalkanes of the formula Ixe2x80x94(CF2CF2)nxe2x80x94I, wherein n is an integer from 2 to 6.
The present invention is a process for manufacturing xcex1, xcfx89-diiodoperfluoroalkanes from 1,2-diiodoperfluoroethane and tetrafluoroethylene (TFE). The xcex1, xcfx89-diiodoperfluoroalkanes made by this process are of the formula Ixe2x80x94(CF2CF2)nxe2x80x94I, wherein n is an integer from 2 to 6.
In the process of the present invention, 1,2-diiodoperfluoroethane is first heated in a reactor to a temperature sufficient for it to thermally decompose to the Ixe2x80x94(CF2CF2)xc2x7radical. This temperature is typically in the range of 200xc2x0 to 240xc2x0 C., preferably 210xc2x0 to 235xc2x0 C. and most preferably 220xc2x0 to 230xc2x0 C.
A molar excess quantity of tetrafluoroethylene is then fed to the reactor so that the total reactor pressure is in the range of 1.7 to 3.4 MPa (preferably 2.1 to 3.1 MPa, most preferably 2.6 to 2.9 MPa). The temperature and pressure is maintained in this range for a period of time, typically 5 to 20 hours to allow the telomerization reaction to take place, forming xcex1, xcfx89-diiodoperfluoroalkanes.
During this reaction period, a competing reaction forms the byproduct perfluorocyclobutane via dimerization of TFE. The perfluorocyclobutane inhibits the formation of the xcex1, xcfx89-diiodoperfluoroalkanes. Applicants have found that removal of the perfluorocyclobutane (i.e. TFE-dimer) from the reactor throughout the reaction period, greatly improves the conversion of 1,2-diiodoperfluoroethane. Thus, at least once (preferably at least twice, most preferably at least three times) during the reaction period, the mixture in the reactor is cooled to a temperature below 75xc2x0 C. so that the unreacted 1,2-diiodoperfluoroethane and xcex1, xcfx89-diiodoperfluoroalkanes are in the liquid state. Unreacted TFE and gaseous perfluorocyclobutane are then discharged from the reactor.
After each time that perfluorocyclobutane has been discharged, the liquid mixture remaining in the reactor is re-heated to a temperature between 160xc2x0 and 235xc2x0 C. The reactor is charged with a molar excess of TFE so as to result in a total reactor pressure between 1.7 and 3.4 MPa (preferably 2.1 to 3.1 MPa, most preferably 2.6 to 2.9 MPa) and the telomerization reaction allowed to proceed.
Optionally, the process of this invention may begin with the reaction of iodine and TFE to form 1,2-diiodoperfluoroethane, which then thermally decomposes to form the Ixe2x80x94(CF2CF2)xc2x7radical and the process proceeds as above.